It is known that suitable refractory systems for the investment casting of reactive alloys such as titanium, zirconium, etc., are very difficult to achieve due to the extremely reactive nature of such alloys. These alloys will reduce most oxides on contact during the casting process yielding extensive gas defects and causing solubility of oxygen in the surface layers of the casting thus requiring extensive chemical machining to remove this layer as it is brittle and renders the castings unsuitable for use in most applications. This is a very serious problem in castings being used in aerospace applications for example. Current yttria systems have been developed and are being used commercially, but these systems depend on short-lived prime coat slurries, which must be carefully controlled and can only be maintained for one week or less. Thereafter, such slurries tend to severely degrade.
In the yttria prime slurries currently used in the marketplace, the approach to overcoming the tendency for ions liberated by the relatively rapid dissolution of yttria in lower pH condition is one of two general methods. In one, additions of a large organic bases such as tetraethylammonium hydroxide for example to a yttria slurry helps prevent the dissolution of silica at the high pH levels necessary to keep the slurry stable. Other approaches have been to “alloy” the yttria with other non-reactive oxides in a fusion process which tends to reduce the number of dissolution sites on the yttria particles or to coat the yttria with large adsorbed organic molecules to achieve the same result, both methods contributing to the overall stability of the system. While these methods work to some degree, there is still a need for reactive alloy mold systems that require limited amounts, if any, of such additives and can utilize commercially available refractory materials such as pure fused yttria or sintered yttria material to produce a stable slurry with a relatively long shelf life. In the present disclosure, such a system is presented.